Organic semiconductor device performance improvement

On a side note, Ouskova and co-workers also reported that the composite of magnetic /i-FejOs nanorods in 5CB showed lower threshold voltages than pure 5CB, and that the sensitivity of the nematic liquid crystal to external magnetic fields was increased in the presence of such magnetic nanorods [451]. Finally, several groups interested in the macroscopic organization and orientation of nanorods also reported on the formation of a lyotropic liquid crystal phase induced by the self-assembly of polymer-coated semiconductor nanorods [453—457], which might be used to improve the device performance, for example, of solar cells. 

Functionalization of pentacene with the specific aim of improving performance in devices is a recent endeavor - the first use of a functionalized pentacene in a field-effect transistor was reported only recently (2003) [26], Functionalization of pentacene has led to the ability to engineer the solid-state arrangement, electronic, and solubility properties of this important semiconductor and to improve its stability and film-forming ability. Recent functionalized pentacene materials have yielded devices with properties comparable with those of the parent acene, have enabled the formation of devices from solution-deposited films, and have even changed the semiconductor behavior of this organic molecule from p-type to n-type. As functionalization strategies are refined, materials with all of the properties necessary for commercial device applications should soon be developed. 

A new class of semiconductors based on a tertiary diamine structure, 5,11-disub-stituted indolo[3,2-/ ]carbazole (15a-15d) (Figure 3.1.5) have been reported.[109,l 10] The derivatives are soluble in organic solvents such as toluene, chloroform, and chlorobenzene. OTFTs using these carbazole derivatives (15a-15d) exhibited p-chan-nel behavior with mobilities of up to 0.12 cm V s and a current on/off ratio of 10 . Recently dichloro derivatives (15e-15f) of these carbazoles gave improved device performance with mobility of 0.14 cm V s . [lll] 2,7-Carbazolevinylene-based conjugated oligomers, 15h-15j, have also been tested in OTFTs.[112,113] The higher mobility of 15h (0.30 cm V s with on/off ratio of 10 ) compared to 15J (10 cm V s with on/off ratio of 10 ) is attributed to the relative coplanar structure of the former and twisted biphenyl units in latter. 

With a view to improving device performance, Kirwan and co-workers proposed the polysilicon self-aligned gate process in 1969. This process not only improved device reliability, it also reduced parasitic capacitances. Furthermore, in 1969, the metal-organic chemical vapor deposition (MOCVD) process was developed hy Manasevit and Simpson, which found widespread adoption in the fabrication of compound semiconductors such as GaAs. 

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